134 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. \Yol.Y. 



been preserved in a fossil state, into tlie cliitinous, siliceous, and calca- 

 reous. Chitiaous skeletal substances are almost wholly, if not entirely 

 confined to the Articulata (including the lyopomatous brachiopods) 

 and certain Acalephsj the siliceous, excepting the teeth of certain 

 moUusks, to the Protozoa ; while, with the exception of aU but a few 

 of the Articulata, all other invertebrates possess skeletal parts, if any, 

 which are composed of calcareous substance only as their mineral 

 constituent. These remarks, of course, apply to the composition of those 

 skeletal parts in their living state. The mineral change which some of 

 them have undergone after their first fossilization will be considered in 

 following paragraphs. In fact, then, we find that the original mineral 

 composition of far the greater part of invertebrate fossils was, in the 

 living state, lime-carbonate combined with a greater or less proportion 

 of animal substance ; ha^dng been the same indeed as that of the shells 

 and corals which constitute their present living representatives. Since 

 the animal substance of these bodies, as is also the case with bones, was 

 displaced during the process of fossihzation, usually by the infiltration 

 and precipitation of additional lime-carbonate, it would seem that the 

 chances for, and the condition of, the preservation of all kinds of these 

 calcareous skeletal remains would be equal in identical cii-cumstances of 

 environment. But there is, as we shall see, a greater variety of condition 

 among fossil forms of this kind than is referable to either their original 

 mineral composition or the environing circumstances of their fossiliza- 

 tion and preservation. 



Three divisions may be made of the manner in which fossil forms are 

 preserved, namely : (1) fossilization proper of the substance of the skeletal 

 parts j (2) moulds 5 (3) casts; (4) pseudomorphs. In the first case, the ori- 

 ginal substance, as well as the external form of the object, is more or less 

 completely preserved. In the second, the whole substance of the object, 

 at some time subsequent to its roclsy entombment, has been removed by 

 decomposition or solution, and its consequent escape through the porous 

 imbedding matrix, leaving a cavity which is the exact counterpart of 

 the external form of the formerly inclosed object. In the third case, the 

 more or less complete external form of the object has been restored by 

 the refilling of the mould with a substance similar to that of the matrix, 

 ■or the deposition of some mineral either wholly or in part different from 

 the matrix; such as calcite, pyrite, &c. In the fourth case, that of 

 pseudomorphs, the substance of the fossil in its original (jondition has 

 been exchanged, atom by atom, for another mineral substance, usually 

 .silex. 



Pseudomorphs thus resemble casts, and in one sense they are such, 

 but they differ from casts, which consist merely of the filling-substance 

 of previously formed moulds, by the manner of their production, and in 

 many cases, at least, by having the texture and even the microsco- 

 pic structure of the original body preserved. This method of preser- 

 vation is perhaps more clearly exemplified in the case of silicified 



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